A battery protection device protects a battery from an abnormal state. The battery protection device includes a detector that detects an abnormality of the battery and outputs an abnormality detection signal when the abnormality is detected. A current is input to an input terminal from outside. A voltage converter converts the current input to the input terminal to voltage and outputs the voltage. A combiner combines the abnormality detection signal with the voltage and outputs a combined signal. A current converter converts the combined signal to a current. An output terminal outputs the current.